Synthesis and Coordination Properties of Quinoline Pendant Arm Derivatives of [9]aneN3 and [9]aneN2S as Fluorescent Zinc Sensors

Neutral Ligands as Potential 64Cu Chelators for Positron Emission Tomography Imaging Applications in Alzheimer's Disease

Positron emission tomography (PET), which uses positron-emitting radionuclides to visualize and measure processes in the human body, is a useful noninvasive diagnostic tool for Alzheimer's disease (AD). The development of longer-lived radiolabeled compounds is essential for further expansion of the use of PET imaging in healthcare, and diagnostic agents employing longer-lived radionuclides such as ⁶⁴Cu (t_1/2 = 12.7 h, β⁺ = 17%, β^- = 39%, electron capture EC = 43%, and E_max = 0.656 MeV) can accomplish this task. One limitation of ⁶⁴Cu PET agents for neuroimaging applications is their limited lipophilicity due to the presence of several anionic groups needed to ensure strong Cu chelation. Herein, we evaluate a series of neutral chelators containing the 1,4,7-triazacyclononane or 2,11-diaza[3.3](2,6)pyridinophane macrocycles that have pyridyl-containing arms incorporating Aβ-peptide-interacting fragments. The crystal structures of the corresponding Cu complexes confirm that the pyridyl N atoms are involved in binding to Cu. Radiolabeling and autoradiography studies show that the compounds efficiently chelate ⁶⁴Cu, and the resulting complexes exhibit specific binding to the amyloid plaques in the AD mouse brain sections versus wild-type controls.

A hemicyanine-based "turn-on" fluorescent probe for the selective detection of Cu2+ ions and imaging in living cells

A novel hemicyanine-based fluorescent probe was developed for the fast, selective, and reversible detection of Cu²⁺ ions with "turn-on" fluorescence response in the aqueous solution. The sensing mode was demonstrated through fluorescence, UV-vis, HRMS, ¹H NMR, and DFT calculations. The probe was successfully used for the imaging of Cu²⁺ ions in living cells.

Aza- and Mixed Thia/Aza-Macrocyclic Receptors with Quinoline-Bearing Pendant Arms for Optical Discrimination of Zinc(II) or Cadmium(II) Ions

The synthesis and coordination properties of two fluorescent chemosensors, featuring [9]aneN₃ (1,4,7-triazacyclononane; L1) and [12]aneNS₃ (1-aza-4,7,10-trithiacyclododecane; L2) as receptor units, and a quinoline pendant arm with an amide group as a functional group spacer are described. The optical responses of L1 and L2 in the presence of several metal ions were analysed in MeCN/H₂ O (1 : 4 v/v) solutions. A selective chelation enhancement of fluorescence (CHEF) effect was observed in the presence of Zn²⁺ in the case of L1, and in the presence of Cd²⁺ in the case of L2, following the formation of a 1 : 1 and a 1 : 2 metal/ligand complex, respectively, which was also confirmed by potentiometric measurements. ¹ H and ¹³ C NMR measurements in CD₃ CN/CDCl₃ in combination with molecular mechanics calculations show that for both complexes of L1 and L2 with Zn²⁺ and Cd²⁺ , respectively, the coordination of the carbonyl group from the pendant arm could be the origin of the observed optical selectivity.

Chemosensing of Guanosine Triphosphate Based on a Fluorescent Dinuclear Zn(II)-Dipicolylamine Complex in Water

Guanosine triphosphate (GTP) is a key biomarker of multiple cellular processes and human diseases. The new fluorescent dinuclear complex [Zn₂(L)(S)][OTf]₄, 1 (asymmetric ligand, L = 5,8-Bis{[bis(2-pyridylmethyl)amino] methyl}quinoline, S = solvent, and OTf = triflate anion) was synthesized and studied in-depth as a chemosensor for nucleoside polyphosphates and inorganic anions in pure water. Additions at neutral pH of nucleoside triphosphates, guanosine diphosphate, guanosine monophosphate, and pyrophosphate (PPi) to 1 quench its blue emission (λ_em = 410 nm) with a pronounced selectivity toward GTP over other anions, including adenosine triphosphate (ATP), uridine triphosphate (UTP), and cytidine triphosphate (CTP). The efficient quenching response by the addition of GTP was observed in the presence of coexisting species in blood plasma and urine with a detection limit of 9.2 μmol L^-1. GTP also shows much tighter binding to the receptor 1 on a submicromolar level. On the basis of multiple spectroscopic tools (¹H, ³¹P NMR, UV-vis, and fluorescence) and DFT calculations, the binding mode is proposed through three-point recognition involving the simultaneous coordination of the N⁷ atom of the guanosine motif and two phosphate groups to the two Zn(II) atoms. Spectroscopic studies, MS-ESI, and DFT suggested that GTP bound to 1 in 1:1 and 2:2 models with high overall binding constants of log β_1 (1:1) = 6.05 ± 0.01 and log β₂ = 10.91 ± 0.03, respectively. The optical change and selectivity are attributed to the efficient binding of GTP to 1 by the combination of a strong electrostatic contribution and synergic effects of coordination bonds. Such GTP selectivity of an asymmetric metal-based receptor in water is still rare.

Quinoline- and isoquinoline-derived ligand design on TQEN (N,N,N′,N′-tetrakis(2-quinolylmethyl)ethylenediamine) platform for fluorescent sensing of specific metal ions and phosphate species

Utilizing the unique metal-binding and fluorescent properties of methoxy-substituted (iso)quinolines, varieties of fluorescent probes were developed from TQEN (N,N,N′,N′-tetrakis(2-quinolylmethyl)ethylenediamine) structure.

[9]aneN3-based fluorescent receptors for metal ion sensing, featuring urea and amide functional groups

The sensing and recognition properties of three new [9]aneN₃-based chemosensors have been studied both in solution and in the solid state.

Methoxy-substituted tetrakisquinoline analogs of EGTA and BAPTA for fluorescence detection of Cd2+

A 5,6,7-trimethoxyquinoline-based octadentate ligand with a BAPTA (1,2-bis(2-aminophenoxy)ethane-N,N,N′,N′-tetraacetic acid) structure exhibits Cd²⁺-specific fluorescence enhancement.

A Bis-Triazacyclononane Tris-Pyridyl N9 -Azacryptand "Beer Can" Receptor for Complexation of Alkali Metal and Lead(II) Cations

A new bis-triazacyclononane tris-pyridyl N₉ -azacryptand ligand is prepared via a convenient one-pot [2+3] condensation reaction between triazacyclononane and 2,6-bis(bromomethyl) pyridine in the presence of M₂ CO₃ (M=Li, Na, K). The proton, lithium, sodium, potassium and lead(II) complexes of the ligand are characterised in the solid state. Preliminary solution-phase competition experiments indicate that the cryptand ligand preferentially binds lead(II) in the presence of sodium, calcium, potassium and zinc cations in methanol solution.

A zinc fluorescent sensor used to detect mercury (II) and hydrosulfide

A zinc sensor based on quinoline and morpholine has been synthesized. The sensor selectively fluoresces in the presence of Zn²⁺, while not for other metal ions. Absorbance changes in the 350nm region are observed when Zn²⁺ binds, which binds in a 1:1 ratio. The sensor fluoresces due to Zn²⁺ above pH values of 6.0 and in the biological important region. The Zn²⁺-sensor complex has the unique ability to detect both Hg²⁺ and HS^-. The fluorescence of the Zn²⁺-sensor complex is quenched when it is exposed to aqueous solutions of Hg²⁺ with sub-micromolar detection levels for Hg²⁺. The fluorescence of the Zn²⁺-sensor complex is also quenched by aqueous solutions of hydrosulfide. The sensor was used to detect Zn²⁺ and Hg²⁺ in living cells.

Fluorescence sensor for sequential detection of zinc and phosphate ions

A new, highly selective turn-on fluorescent chemosensor based on 2-(2'-tosylamidophenyl)thiazole (1) for the detection of zinc and phosphate ions in ethanol was synthesized and characterized. Sensor 1 showed a high selectivity for zinc compared to other cations and sequentially detected hydrogen pyrophosphate and hydrogen phosphate. The fluorescence mechanism can be explained by two different mechanisms: (i) the inhibition of excited-state intramolecular proton transfer (ESIPT) and (ii) chelation-induced enhanced fluorescence by binding with Zn(2+). The sequential detection of phosphate anions was achieved by the quenching and subsequent revival of ESIPT.

OFF–ON–OFF fluorescent response of N,N,N′,N′-tetrakis(1-isoquinolylmethyl)-2-hydroxy-1,3-propanediamine (1-isoHTQHPN) toward Zn2+

1-isoHTQHPN exhibits OFF–ON–OFF fluorescent response toward increasing concentration of Zn²⁺ions due to the specific intramolecular excimer formation in a mononuclear complex.

Synthesis and structural chemistry of bicyclic hexaaza-dithia macrocycles containing pendant donor groups

An efficient medium-scale synthesis and the structural characterization of a series of macrobicyclic hexaaza-dithia macrocycles with pendant donor arms is presented.

A new high-capacity metal ion-complexing gel containing cyclen ligands

“Hairy” nano-scale objects cross-linked into a high-capacity metal-binding hydrogel.

Quinoline benzimidazole-conjugate for the highly selective detection of Zn(ii) by dual colorimetric and fluorescent turn-on responses

A quinoline benzimidazole-conjugate (QBC) has been synthesized for the highly selective detection of Zn(ii) both by colorimetry and fluorimetry.

A novel turn-on Schiff-base fluorescent sensor for aluminum(iii) ions in living cells

Quinoline Schiff base L exhibits a more than 38-fold fluorescence enhancement after addition of Al³⁺ ions at an excitation wavelength of 373 nm, and shows a high selectivity for Al³⁺ over other metal ions.

Synthesis of N-acetyl-L-cysteine-capped ZnCdSe quantum dots via hydrothermal method and their characterization

Compared with the most studied green-red emitting (530-650 nm) quantum dots (QDs), the preparation of short-wavelength-emitting QDs remains difficult. Besides, one of the representative short-wavelength QDs materials, ZnCdSe, has a shortcoming of high content of toxic cadmium metal. In this paper, we report the synthesis of high-quality water-soluble ZnCdSe QDs via optimized one-step hydrothermal method with a new thiol as ligand, within a short time of 65 min. The emission wavelength of prepared QDs is tunable in the range of 425-540 nm by merely controlling the molar ratio of Cd:Zn or Se:Zn, and the quantum yield reaches 35%. More importantly, the maximum Cd:Zn molar ratio has been reduced to 0.04:1.0, much lower than that reported in the literature (0.5:1.0), resulting in excellent biological compatibility of prepared QDs and thus their promising applications in biological fields. Moreover, the transmission electron microscopy was employed to examine the effect of Cd:Zn ratio on the size of prepared ZnCdSe QDs, which were also characterized by x-ray photoelectron spectroscopy and electron diffraction spectroscopy.

Quinoline-attached triazacyclononane (TACN) derivatives as fluorescent zinc sensors

TACN (1,4,7-triazacyclononane) derivatives with three 6-methoxy-2-quinolylmethyl or 1-isoquinolylmethyl moieties were examined as fluorescent zinc sensors. Upon the addition of zinc, 6-MeOTQTACN (5) exhibited a 9-fold fluorescence increase at 420 nm (λex = 341 nm, ϕZn = 0.070). Fluorescence enhancement is specific for zinc and cadmium, although cadmium induces smaller increases (ICd/I0 = 3.6 and ICd/IZn = 40%). The isoquinoline analog 1-isoTQTACN (6) exhibits minimal fluorescence enhancement upon zinc binding. TPEN (N,N,N',N'-tetrakis(2-pyridylmethyl)ethylene-diamine) does not extract zinc from the 6-MeOTQTACN-Zn complex (5-Zn). The quantum yield, metal ion selectivity and metal binding affinity differences between TACN and ethylenediamine (EN) skeletons in quinoline-based ligands are discussed based on the X-ray crystallographic analysis of zinc and cadmium complexes, demonstrating the superiority of quinoline-TACN conjugates.

Sequential recognition of zinc ion and hydrogen sulfide by a new quinoline derivative with logic gate behavior

A Schiff base-type fluorescent chemosensor L has been synthesized for Zn²⁺ detection, and the consequent product of L and Zn²⁺, L–2Zn, is an excellent indicator for H₂S for displacement of Zn²⁺ from the complex L–2Zn.

Coordination of lead(II) in the supramolecular environment provided by a "two-story" calix[6]arene-based N6 ligand

First insights into the coordination properties and host-guest behavior of a "two story" calix[6]aza-cryptand (1) are described. The ligand is constituted of a triazacyclononane (TACN) cap and three pyridine (PY) spacers connected to the calix[6]arene small rim. The resulting N6 donor site coordinates Pb(II) ions to give complexes that are highly stable. X-ray diffraction structures reveal a hemidirected environment for Pb(II) with strong coordination to the TACN cap and weaker bonds with the three PY residues. A guest molecule, either water or EtOH, sitting in the calixarene macrocycle and hydrogen-bonded to the phenoxyl units at the level of the small rim further stabilizes the complexes through electrostatic interactions with the metal center and the calixarene core. In-depth (1)H NMR studies confirm the host-guest behavior of the complexes in solution, with evidence of embedment of neutral guest molecules such as EtOH, BuOH, and N-Me-formamide. Hence, in spite of the presence of a N6 donor, the calixarene macrocycle can be open to guest interaction, giving rise to seven-coordinate dicationic complexes. Noteworthy also is the flexibility of the macrocycle that allows Pb(II) to adopt its preferred hemidirected environment in spite of the three covalent links connecting the calixarene core to the three PY groups. The flexibility of the system is further illustrated by the possible coordination of an exogenous anionic ligand in the exo position. Hence, compared to the previously described "one story" calix[6]aza-cryptands, ligand 1 displays several similar but also new features that are discussed.

Anion receptor chemistry: highlights from 2011 and 2012

This review covers advances in anion complexation in the years 2011 and 2012. The review covers both organic and inorganic systems and also highlights the applications to which anion receptors can be applied such as self-assembly and molecular architecture, sensing, catalysis and anion transport.

Monopicolinate-dipicolyl derivative of triazacyclononane for stable complexation of Cu2+ and 64Cu2+

The synthesis and characterization of Hno1pa2py, a new tacn-based ligand, is reported. The complexation process with Cu(2+) was proved to be very fast even in acidic medium. Potentiometric titrations allowed us to establish that Hno1pa2py exhibits an overall low basicity as well as a high selectivity for Cu(2+) over Zn(2+) cations. The copper(II) complex was synthesized and characterized using UV-vis and EPR spectroscopies and density functional theory (DFT) calculations. The studies clearly showed that the [Cu(no1pa2py)](+) complex is present in solution as a mixture of two isomers in which the ligand is coordinated to the metal center using a N5O donor set with the metal center in a distorted octahedral geometry. The very high kinetic inertness of the [Cu(no1pa2py)](+) complex was demonstrated by using acid-assisted dissociation assays as well as cyclic voltammetry. Preliminary investigations of (64)Cu complexation were performed to validate the potential use of such chelating agent for further application in nuclear medicine. The X-ray crystal structures of copper(II) complexes of L1, the ester derivative of Hno1pa2py, have been determined.

Zinc-specific fluorescent response of tris(isoquinolylmethyl)amines (isoTQAs)

Isoquinoline-based tetradentate ligands with C(3)-symmetry, tris(1- or 3-isoquinolylmethyl)amine (1- or 3-isoTQA), have been prepared and their zinc-induced fluorescence enhancement was investigated. Upon excitation at 324 nm, 1-isoTQA shows very weak fluorescence (ϕ = ∼0.003) in DMF/H(2)O (1/1) solution. In the presence of zinc ion, 1-isoTQA exhibits fluorescence increase (ϕ = 0.041) at 359 and 470 nm. This fluorescence enhancement at 470 nm is specific for zinc. However, 3-isoTQA exhibited a smaller fluorescence enhancement upon zinc complexation (ϕ = 0.017, λ(em) = 360 and 464 nm) compared with 1-isoTQA. Crystal structures of zinc complexes of isoTQAs demonstrate the diminished steric crowding and shorter Zn-N(aromatic) distances compared with isoTQENs (N,N,N',N'-tetrakis(isoquinolylmethyl)ethylenediamines) leads to a higher fluorescent response toward zinc relative to cadmium.

Identifying three-way DNA junction-specific small-molecules

Three-way junction DNA (TWJ-DNA, also known as 3WJ-DNA) is an alternative secondary DNA structure comprised of three duplex-DNAs that converge towards a single point, termed the branch point. This point is characterized by unique geometrical properties that make its specific targeting by synthetic small-molecules possible. Such a targeting has already been demonstrated in the solid state but not thoroughly biophysically investigated in solution. Herein, a set of simple biophysical assays has been developed to identify TWJ-specific small-molecule ligands; these assays, inspired by the considerable body of work that has been reported to characterize the interactions between small-molecules and other higher-order DNA (notably quadruplex-DNA), have been calibrated with a known non-specific DNA binder (the porphyrin TMPyP4) and validated via the study of a small series of triazacyclononane (TACN) derivatives (metal-free or not) and the identification of a fairly-affinic and exquisitely TWJ-selective candidate (a TACN-quinoline construct named TACN-Q).